Peat is a carbonaceous resource which occurs very extensively in Canada and other circumpolar countries. It is also found in many topical countries where appropriate conditions of high water content and absence of oxygen allow the accumulation of decaying biomass. As a result peat in situ is often extremely wet and may contain less than 5% by weight of solid material. In order to transport and utilize peat, it is necessary to remove the majority of the water as close to the mining site as possible. Generally a solids content of greater than 50% is desired. To put this problem in its most dramatic perspective 18 tonnes of water would have to be removed from 20 tonnes of such peat as mined in order to produce 2 tonnes of 50% moisture content peat. The energy contained in the final product is much less than that required to remove 18 tonnes of water and as a result it is not practical to use direct thermal means involving evaporation of the water to prepare the dry peat.
Peat as found in nature has undergone varying stages of biological decomposition. Peat is formed by microorganisms from plants of lignocellulosic nature. According to the extent of the decomposition it is possible to recognize the nature of the lignocellulosics originally present. It is thus normal to find remnants of hemicelluloses, some sugars, cellulose, lignin, peptides and newly created materials obtained via the microorganisms' action. Metals are normally complexed to these macromolecular structures indicating a possible role during the decomposition.
The structure (three-dimensional) of peat is not yet known with certainty although the following picture should approximate the actual arangement:
a central core of humic materials, highly hydroaromatic, derived most likely from lignin and cellulose degradation;
the core consists of phenolic-like monomers bridged via 0 bonds and adopting, for entropy reasons, a coiled position;
a very large number of OH groups impart their hydroaromatic nature to this core;
attached to this central core via hydrogen-bonds are:
carbohydrates, mainly of cellulosic nature although in some cases the hemicelluloses might have contributions PA1 peptides, contributing a significant percentage to the total N present in peat PA1 inorganic matter either entrapped or ion-exchanged (metals) PA1 resins and waxes derived from the plant liptinitic material and probably absorbed via some H-bonds to the central core PA1 milling peat. PA1 drying peat in an oven. PA1 mixing dried material with concentrated sulphuric acid in a proportion ranging from 1.5 to 3 g H.sub.2 SO.sub.4 /100 g of dry peat. PA1 the mixed material is then extruded through a screw-type reactor where intensive shear forces break down the polymeric structures causing substantial hydrolysis. PA1 the mass then enters a reactor where it is diluted with H.sub.2 O and steam is added to heat the slurry thus formed. The slurry must thus be heated to about 140.degree. C.-190.degree. C. where solubilization of the sheared polymers and its final hydrolysis to monomers is conducted.
The hydrophilic nature of peat should thus be attributed to the extraordinary capacity of the material towards H-bonding.
In well decomposed materials the fibrous character of the cellulose remnants is almost non-existent. The overall peat structure becomes highly colloidal in an environment somewhat acidic whose pH varies between 3 and 6, although normally it is between 4 and 6.
Peat that has only lightly decomposed still contains recognizable plant debris and is often sold as horticultural peat. The von Post scale is used to describe the extent of decomposition and has limits of 1 and 10. Peats for energy purposes are generally of von Post values of 5 or greater. Such peats are very decomposed and have a colloidal structure which makes it very difficult to simply press the water mechanically from the peat and water matrix.
In countries with appropriate climates--long frost free periods, adequate and predictable periods of dry/sunny/windy weather--a solar drying method based on milling or extruding the peat on the prepared surface of the bog is possible. This is practiced on a large scale in the USSR, Finland, Sweden and Ireland. The Canadian peat moss industry uses similar methods but the short seasons can only be justified for a premium product such as horticultural peat moss.
Where climates are more severe and there is an application for peat such as energy which requires almost year round operation, thermal treatments have been proposed which exploit the thermal/mechanical rearrangement of the peat/water matrix in order to have the bulk of the water drain from the matrix by, for example, filtration followed by thermal drying.
Processes of thermal rearrangement and mechanical treatment of peat are known. A semi-commercial plant using such a process for example was in operation around the end of World War I in Dumfries, Scotland.
More recent processes include derived fuel processes (PDF) and the Koppelman process.
The PDF process is a wet carbonization treatment of peat at 200.degree. C., 2.5 MPa and residence times of 30 minutes. Specially designed heat exchangers and multiple-effect condensor-evaporators are used for efficient heat recovery to provide a thermal efficiency estimated between 72% and 80%. The wet carbonized peat slurry is dewatered to 50 weight percent moisture in semi-continuous and fully automatic pressure filters. The filter cake is then pulverized and further dried to about 10 weight percent moisture in a flash dryer. The dried peat is then pelleted and briquetted.
The Koppelman process, described and illustrated in U.S. Pat. No. 4,477,527 issued Oct. 16, 1984, is a two-stage, high temperature, high pressure beneficiation process giving a K-fuel product with a heating value about 50% greater than that of the raw peat. The first stage is a wet carbonization under conditions similar to the PDF process followed by filtration to 70 weight percent moisture. The dewatered peat is conveyed by a twin-screw feeder to the second stage operating at 400.degree. C. and 10.4 MPa for a residence time of 10 minutes. The peat is extruded and cooled giving a K-fuel product with less than 10 weight percent moisture.
A useful criterion for assessing the utility and practicality of such processes is a measurement of the "severity" of the process. The parameter that reflects severity of such a process is a "P" factor which is calculated as follows: ##EQU1## Where T reaction is the reaction temperature of the peat.
The severity of a typical PDF process and a typical Koppelman process is set out in Table I hereafter.
TABLE I ______________________________________ Time Mass Heating "P Factor" Process Temp/C. Min. Yield % Ratio* min ______________________________________ PDF 210 30 60 1.21 52000 Koppelman 320 15 40 1.22 450000 ______________________________________ *The heating ratio is the ratio of the higher heats of combustion of the product over those of the input peat.
It can be seen from the above Table that the existing PDF and Koppelman processes use long duration treatments and obtain a dewaterable product in reduced yield due to their high severity. This causes solubilization of a large amount of the peat and this in turn is costly with respect to the water treatment. The long duration treatment results in low throughput unit of investment.
Other processes of dewatering peat, either being wet carbonaceous processes or processes using heat and high-pressure in combination, are described and illustrated in U.S. Pat. No. 2,573,134 of Gebaurer issued Oct. 30, 1951, U.S. Pat. No. 2,668,099 of Cederquist issued Feb. 2, 1954, U.S. Pat. No. 4,153,420 of Myreen issued May 8, 1979 (corresponding to Canadian Patent No. 1,119,407 issued Mar. 9, 1982) and Canadian Patents Nos. 188,789 of ten Bosch issued Feb. 18, 1919 and 208,415 of ten Bosch issued Feb. 8, 1921. Again, such processes use long duration treatments and have high severity.
Finally, a recently developed Russian hydrolyzer process has been used in the production of sugars (apparently fermented to single cell protein). This process is not intended for dewatering of peat. This process, developed at the Institute of Wood Chemistry in Riga, is intended to process peat (slightly decomposed i.e. von Post 1-2 humification) so that a fermentable juice for the production of a single cell protein is obtained. That process involves:
A disadvantage of the Russian technology is that it requires drying the initial peat which makes it unsuitable as a dewatering process.
Modification of the Russian technology to process wet highly decomposed material seems improbable since, once macerated, the wet material acts as a fluid not permitting any extrusion.
From the Russian experience, however, it is realized that extensive hydrolysis is possible and relatively easy to carry out.